Filter for periodic electric currents



June '21, 192922 M. K. ZNN

FILTER ma PERIODEQELEGTRIC GURRENTS l5, 1922 2 Sheets-Shes*v l Filed Deo.

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M. K. ZlNN FILTER FOR PERIODIC ELECTRIC cUaRBNTs Filed Deo. l5. 1922 2 Sheets-5heet 2 www jy Patented June 21,1927.

UNITED 'STATES im. I.

line-'iniziai 'oi-Fics FILTER FOB PERIODI!) ELECTRIC CURRENTB.

Application illed December 15, 1822.* Serial No. 607,182.

An object of my invention is to provide a new and improved electric current filter of the type having recurrent sections. Another object of myinvention is to provide such a l filter having an advantageous attenuation characteristic andof simple and practicable structure. Another object is with simple structure to provide a filter with a sharp cut-off between the attenuated and transill mitted zones of frequency. A further object is to do thiswith sustained high attenuation throughout the attenuated zone. Another object is to reduce the attenuation losses to a negligible amount within the transmitted zone of frequencies. All these and other objects of my invention will be made apparent in the' following specification and claims, taken with the accompanying drawings, wherein I have disclosed -a limited number 2 of specific embodiments of the invention by way of illustration and example. Itl will be understood that the invention is defined in the appended claims.

- Referring to the drawings Figure 1 is a general dia am of a filter having mutual impedance Fig. 2 is a filter like Fig. 1, with position ofadditicnal shunt elements. Fig. 3 is a special case of Fig. 1 inv which the impedance .elements are shown as coils' and condensers. Fig. 4 is a special case of Fig. 2, with the same .qualification 5 is'a'nl other special case of Fig. 1. ig. 6 is a. corresponding special case of Fig. 2. Fi s. 7, 8, 9 and 1U are characteristic diagrams or Figs. 3, 4, 5 and 6, respectively, showin attenuation as a function of frequency. igs. 11, 1'2, 13 aiidllv are diagrams offilters that will be referred .to -for purposes of comparison, and Figs. 15, 16, 17 and' 18 are respective characteristics for lthese filters.

The properties of an electric wave filter are most easily demonstrated by first assuming that the structure consists of an infinite number of sections, and that the coils and condensers have no powerlosses.y The performance of an actual filter will de art from this ideal case by a small amount epending upon the number of sections employed the design of the terminal impedances, and the efficiency of the circuit elements.

etween certain series elements. the .inter-y zum, or Baoonnm, naw Yoan, assroNoa iro American mariachi nm rameaux coursier, a coaroaarroN or New Your.

In .an artificial'line comprising a very large number of identical sections the current decreases exponentially from one sec-` tion to the next, that is:

Ninereases in the direction of propagations` The performance of the filter is therefore completely determined by. I, the propagation constant per section.. v.

The formulae given in the following assume the ideal case of au infinite number of sections and no/power losses.

alsl

' For the filter shown in Fig.'3', whose at-` tenuatoii characteristic is sketched in Fig. 7, the (propagation constant per section is define by i where 12:21). l

Thisexpression shows that there isa range of free transmission from 0 to fo, where f ..1 amer O. L.+M l Frequencies higher than f will be suppressed in varying degrees, the attenuation becoming infinite at a frequency l fr V As the frequency is increased yabove fr, the

attenuation decreases, approaching a constant value,

cosh-1%, at very high frequencies. The ratio JC-2 which is a iii-ensure of the discriminatiomis a function of the coetiicient of coupling,

alsV

ico

. There is also a l infinity to y of a single coil with a ta The discrimination` becomes sharper as 1c approaches l, but the attenuation falls B' more rapidly beyond f, and approachesa lower final value.

To get the same selective characteristic without mutual inductance would require three impedance elements per section as for example in Fig.4 13, whereas the filter shown discussed above that the coils have been moved half a section so that they straddle the bridging points. The cut-off frequency of the new circuit differs fromthat of the simple filter having the same sized elements by the factor, y. l

i The filter of Fig. 4 may be designed to' give the attenuation characteristic of Fig. 8. It differs from Fig. 3 only in theaddition of the extra capacity' C3.. This strucvtule may be considered structure b defining a section as the circuits incluc ed betweentwo of thel bridged capacities G3; that is, the minated at either end in one-half C3. With the unit section so specified,

ran es of free transmission. Zero' is one of the order frequencies and the others are:

1 2(C +C) fura/@furie This filter will, inigeneral.

frequency of infinite attennation, viz: A

t21u/Mor 1. The circuit constants maybe so proportioned as to give a acteristics, and fr may be made to fall within either of the opaque bands. If the network is intended to function as av low-pass filter, however, the characteristic of greatest importance is thatI shown in Fig. 8 where The two free rangesare merged into one by makingY flzf, which voceirs if C2 14k "l (ifm.

vAs the frequency increases yfrom f, to infinity, the attenuation first ,decreases from a certain minimum and then rises great variety of selective char-` l-plLi-(CMCu-MCAH i y again, the `network` becoming completely opa ue at .very high frequencles.

.T is filter is therefore capable of very vsharp discrimination and at the Sametime attenuates currents whose fre uen-4 greatly cies are remote from the transmitted 'of fre uencies.

The igh-pass filters in Figs. 5 and 'hiwe attenuation characteristics correspondin exactly to those of Figs. 3 and 44. They iffer only in being reversed.y The high-pass characteristics are sketched in Fi s. 9 and 10.

For the high-pass filter 0 Fig. 5 it can be shown that 201e 41,211.0.) parues CMU 'PLzCx) *PzLzi M The frequency of cut-ofi' is and and the frequency of maximum attenuation is The discrimination is given by propagation as a singly periodic unit section 1s ter-` consists merely in the fact of' Fig. 6 has,f7inl general, af' constant per Section defined It Will function as a. high-class lter if the elements are so proportioned that For `this design the frequency of cut-off is The discrimination is given by less than that required for the ordinary tB-ele-` ment filter without mut-nal impedance and having the same characteristic as regards` cut-off point and discrimination. Such a 3-element lter is shown for example in Fi ,1. 14. The resistance dissipation is, therefore, less, which gives a higher attenuation in the suppressed region, particularly Lat the fre quency et maximum attenuation. The loss in the transmitted range is also smaller for the sume reason. y

In the practical application of wave filters a knowledge of their impedance characteristics is required. Expressions for the impedance ofthe filters heretofore described Will 'now be given. It is found that the impedance of these filters varies with the frequency in exactly the same manner as docs the impedance of simple Z-element or B-element high and vlow lpass filters. In fact the impedance may he made to coincide with the impedancecharacteristics of the ordinary types of filters, which makes it practicable to use these filters .in conjunction with the other types.

Only one termination of these filters will he considered, namely, at a, point between two of the T network structures- When the extra impedance Zn has been added asin Fig.

4 and Fig. 6, the filter will be considered terminated in 22:22,. Other terminations might be devised which would improve the impedance characteristics as has been done for the ordinary types of filters.

The impedance of the filter of Fig. Z2 when terminated as defined above is:

Jiilil). pan s ivn (L, i# My or in terms of The impedance is a pure resistance in the free range approaching,Y a value :ily zero frequency. It becomes zero attliey cnt-ofl` point and is a pure inductive reactance iii the suppressed range. In particular, at. the 'frequency of maximum attenuation it is F rom the above description it will be reeog nized that this filter has the same impedance (-lnirin-teristif' as either a simple or B-elenient filter with mid-series termination. yIt can be made to have exactly the same impedance at. all trctpiciicics as the simple filter having the saine cnt-oft point by designing it so that if L and C arethe elements of the simple filter The discrimination is leftf free toy choice.

The filter of Fig. 4V is terminated in Q; and

has thc saine impedance characteristic as the midshnnt impedance ot a simple or 3element filter.

fThis applies to the design Where ppzpg. As

Stated above this is brought about by making Uzzlk The impedance approaches a constant value at zero frequency, viz:

It is a pure resistance in the free-range, becoming infinite at the cut-ofi' point, p0. In

the suppressed range beyond p., it is a, pure (tft -iie The impedance at any frequency is givenl by y lao y inductive reactence, falling to zero at very '12er-pagg..

high frequencies.

The impedance of the structure of Fig. 5 n

is given by Like the corresponding low pass filter of Fig. 3, the impedance is a resistance in tl1e` ,l at the point o cut-oil.

I Z yis infinite'at zero frequency.

free range and a positive reactance in the suppressed ran e and passes through zero At `very high frequencies 2113 Z approaches m is fixed by the relation,

u Other values of interest are:

r C L r :si I Llm (pi 7JeCNCIl'2C1a) This filter may be made to have the same impedance as the mid-series impe-dance' of i either the simple or 3-e1en1ent high pass filter by proper proportionment of the circuit constants. c

As pointed out before, the design of particular Vimportance for the filter of-Fig. 6

f The structureg-is considered terminated inI 2L3. `lVith this 'termination The impedance characteristic is similar to that of the high pass simple or -element filter when terminated at mid-shunt. `That is, Z isa resistance in the free range and a` positive reactance inthe suppressed range and has an infinite value at the cut-off point.\ l At zero frequenc the impedanceis zero, and at very hlvhfi'equencies it approaches the constant va ue.

, l] C A u Z @racines-cn I claim: i

1. "A filter having successive shunt impedance elements and series elements 1n alternationtherewith, adjacent series elements having mutual impedance in pairs.

2. A filter having successive shunt reactance elements of the same algebraic sign and alternately" arranged "series reactance elements of opposite sign, saidscries elements having mutual reactance in airs. i

3. A filter having series impe-dance elements and shunt impedance 4elements arranged successively in alternation, the" shunt I elements being alternately of ,two values and 'the series elements having mutual. imped' fraisse-nine non values, oneuvalue when they stan-d between series elements having mutual impedance connection and another value when they Another convenient point for computation n saidv series elements having 1n pairs, sharp cut-v stand bet-Weenimpedance elements not so.

connected. y

6. A filter having successive series impedances with mutual impedance connection be-`` tween adjacent impedances in pairs andl having shunt impedances-respectively between the members of each having means to hold up the attenuation to a sustained value throughout the entire attenuating range of the filter.

VIn testimony whereof, I have signed mv December, 1922. 4

MANVEL K. ZINN.V

series pairl and name to this specification this 13th day of I' 

